Chlamydia trachomatis (CT) is the most common bacterial sexually transmitted infection in the United States. Infection with CT usually remains localized to the lower genital tract where it can produce urethral or cervical discharge. However, if left untreated, the infection can ascend to the upper genital tract producing salpingitis, perihepatitis and pelvic inflammatory disease (PID) in women. The resultant tissue damage from the inflammatory response is believed to result in fallopian tube scarring, tubal infertility and ectopic pregnancy. In spite of its clinical importance, little is known about the early immune responses to chlamydia. An investigation into the molecular basis of cellular activation by CT, including a characterization of the innate immune receptors and secondary mediators that initiate and coordinate the subsequent inflammatory response, has never been undertaken. We hypothesize that the interactions between specific chlamydial ligands and their innate immune receptors leads to the development of localized inflammation in the genital tract, and therefore the subsequent tissue damage associated with PID. The goals of this application are to identify the specific cellular receptors and adaptor proteins that are responsible for creating an inflammatory environment during chlamydial infection, and to determine the mechanism by which the host inflammatory response is initiated during infection of both epithelial cells and monocytes/macrophages. We will focus on defining the role of the Toll-like receptors (TLRs) and their adaptor proteins in chlamydial pathogenesis, and have proposed the following specific aims: (1) To characterize the interactions between TLRs and CT during a productive infection of epithelial cells and phagocytic cells; (2) To identify the signaling pathways activated in epithelial cells and phagocytic cells during CT infection; (3) To determine the cellular changes that follow primary infection with CT. These studies should yield novel insights into the pathogenesis of chlamydial infections, as well as the innate immune defenses of the lower female genital tract. A better understanding of the immune response in this compartment could pave the way for the development of better therapeutic agents and mucosal vaccines to combat the immediate and long-term consequences of chlamydial infections.